EP3109339B1 - Method for treating a workpiece made of tantalum or a tantalum alloy - Google Patents

Description

TECHNICAL AREA

The present invention relates to the treatment by cementation of a metal piece made of tantalum or tantalum alloy, in order to make it more mechanically and chemically resistant.

In particular, the process according to the invention makes it possible to form on the surface of a tantalum or tantalum alloy part one or more layers of tantalum carbide, by controlling their structures and their thicknesses.

The fields of application of such a method are numerous and are all areas requiring the production of resistant parts made of tantalum or a tantalum alloy (production of crucibles for metallurgy, electrodes, lamp filaments, resistors , tools, etc.).

STATE OF THE PRIOR ART

Tantalum is a material that is extremely resistant to corrosion and has a very high melting point (melting point ≥ 3000 ° C). The tantalum or tantalum alloy parts are therefore used in many fields and in particular for the manufacture of crucibles for use in pyrochemistry.

In order to make these parts even more resistant to corrosion and increase their hardness, it is possible to make them undergo a cementation, that is to say a thermochemical treatment which consists in increasing the surface carbon content of the part. Subsequent treatment (chemical, mechanical or thermal) can then be implemented to obtain a particular surface microstructure.

There are three types of carburizing according to the state of the cementing medium, namely solid carburizing, liquid carburizing and gaseous carburizing. Among these three types of cementation, four major cementation methods (mainly developed for steels) are commonly described in the literature, namely case carburizing, carburizing under controlled atmosphere, carburizing under reduced pressure and plasma-aided cementation.

In case cementation, the part to be cemented is put directly in contact with solid carbon. Once sublimed, the carbon becomes gaseous solid will be adsorbed on the surface of the room, then spread in the room to react with tantalum. This case cementation method requires a sufficiently high carbon vapor pressure for the tantalum to be cemented properly, which requires very high carburization temperatures (> 2000 ° C) and a long heating time (for example 10h at 1700 ° C in the document [1] ). In addition, this method requires pressing carbon powder on the surface of the part to be cemented and is therefore not applicable to parts having a complex geometry. In addition, due to the solid / solid interface, the carbon contribution on the surface is heterogeneous.

Controlled atmosphere carburizing consists in placing the part to be cemented in a controlled atmosphere furnace, heating the furnace to a carburising temperature (> 1200 ° C for tantalum), then injecting under a pressure of approximately 1 bar a mixture of inert gas (argon) and fuel gas (in general, a hydrocarbon such as methane, acetylene, propane, etc.). In some applications, an air / methanol or nitrogen / methanol mixture may also be employed. The fuel molecules then come to crack on the surface of the part to be cementized and release their carbon, which diffuses then reacts with surface tantalum. This method of carburizing, however, has the disadvantage of generating oxides when an oxygenated compound is injected. In addition, when a hydrocarbon is used, it is common that soot form inside the enclosure of the furnace, polluting it and disrupting the cementation of the room.

Cementation under reduced pressure (also known as low-pressure carburizing) involves placing the part to be cemented in a thermochemical treatment furnace and then evacuating the furnace chamber. The chamber is then heated until the carburising temperature is reached, and then a gaseous hydrocarbon (methane, acetylene, propane, etc.) is injected under a low pressure (that is to say a pressure of less than 100 mbar). from a few millibars to a few tens millibars). This method is known for effectively cementing very complex geometry parts and reduces the pollution of parts. It is this cementation method that we will use in the context of the present invention.

Finally, plasma-assisted cementation is very close to carburizing under reduced pressure. The main interest of this technique lies in the creation of a plasma around the room to be cementer. This plasma activates the surface of the material and thus facilitates the diffusion of carbon in the room. This method is useful for the cementation of very complex geometry parts. However, it remains undeveloped compared to carburizing under reduced pressure because it requires the use of very specific equipment. One of these main drawbacks is that it does not make it possible to treat parts having singularities such as holes of small diameters, these singularities being able to generate a hollow cathode phenomenon (local melting inside the hole). In addition, the bearing face of the parts on the basket of the treatment furnace is never treated, since it is never in direct contact with the plasma.

These four cementation methods make it possible to obtain a heterogeneous structure, composed on the surface of a layer of TaC, then, while approaching the core of the part, an underlayer of Ta ₂ C and finally a layer carbon-saturated tantalum, optionally having Ta ₂ C precipitates at the grain boundaries according to the degree of carbon saturation of the tantalum (a layer which we will also call "saturated Ta layer C" or, if it has precipitates of Ta ₂ C at the grain boundaries, "saturated Ta layer C + Ta ₂ C"). The greater the carbon enrichment, the greater the thickness of the TaC layer will be relative to the thicknesses of the layer of Ta ₂ C and the saturated Ta layer C (or Ta saturated C + Ta ₂ C). If the carbon enrichment is sufficiently large, it is thus possible to convert the tantalum of the tantalum carbide piece TaC completely.

Whatever the cementation method used, one always obtains a layer of TaC on the surface of the part. However, for some applications, it is not desirable to have such a layer on the surface of the part and it is then necessary to remove it.

To remove this superficial layer, chemical surface treatment can be carried out by acid etching. By way of example, such a surface chemical treatment is described in document [1] . The disadvantage of surface chemical treatments is that they modify the surface condition of the parts and are difficult to implement because of the properties of high hardness and high chemical inertness of carbides vis-à-vis acids. It is therefore necessary to use very powerful acid mixtures (the most common being a mixture of nitric, hydrofluoric and lactic acids), which are generally toxic and very dangerous to use. In addition, the etching will attack all of the carbide layers (TaC layer and underlying Ta ₂ C layer) and not only the TaC surface layer, leaving only the layer having a saturated tantalum structure. carbon with Ta ₂ C at the grain boundaries.

STATEMENT OF THE INVENTION

The invention aims to at least partially solve the problems encountered in the solutions of the prior art.

1. a) placer la pièce dans un four et chauffer le four sous vide à une température au moins égale à 1400°C ;
2. b) former un multicouche carboné dans la partie périphérique de la pièce, par injection, dans le four chauffé, d'une source carbonée gazeuse à une pression au plus égale à 10 mbar, le multicouche carboné comprenant au moins une couche C1 en carbure de tantale, qui est située à la surface de la pièce, et deux couches sous-jacentes C2 et C3 comprenant chacune une teneur en carbone différente et inférieure à la teneur en carbone de la couche C1 ;
3. c) stopper la formation du multicouche carboné par refroidissement de la pièce ;
4. d) placer autour de la pièce un dispositif capable de piéger le carbone, l'oxygène et l'azote pour protéger la pièce du carbone ainsi que des éventuelles traces d'oxygène et d'azote présent(s) dans le four ;
5. e) provoquer la diffusion de tout ou partie du carbone présent dans la couche C1 en direction des couches C2 et C3, par chauffage du four sous vide, la pièce étant maintenue dans le dispositif de protection ; et
6. f) stopper la diffusion du carbone dans la pièce par refroidissement de la pièce sous vide avant que le carbone présent dans le multicouche carboné n'atteigne la partie centrale de la pièce ;

To do this, the subject of the invention is a method for treating a tantalum or tantalum alloy part, comprising the steps of:

1. a) place the piece in an oven and heat the oven under vacuum at a temperature of at least 1400 ° C;
2. b) forming a carbonaceous multilayer in the peripheral part of the part, by injecting, in the heated furnace, a gaseous carbon source at a pressure of at most 10 mbar, the carbonaceous multilayer comprising at least one layer C1 of carbide of tantalum, which is located on the surface of the part, and two underlying layers C2 and C3 each comprising a different carbon content and lower than the carbon content of the layer C1;
3. c) stop the formation of the carbonaceous multilayer by cooling the part;
4. d) place around the room a device capable of trapping carbon, oxygen and nitrogen to protect the carbon part as well as any traces of oxygen and nitrogen present in the furnace;
5. e) causing the diffusion of all or part of the carbon present in the layer C1 towards the layers C2 and C3, by heating the oven under vacuum, the piece being maintained in the protective device; and
6. f) stopping the diffusion of carbon into the room by cooling the vacuum chamber before the carbon in the carbonaceous multilayer reaches the central part of the room;

whereby a room is obtained whose surface is free of tantalum in the form of TaC, the central portion of which is free of carbon and whose part (hereinafter "intermediate part") situated between the surface and the central part, includes tantalum and carbon.

In the method that is the subject of the invention, the diffusion in step e) causes the decomposition of all or some of the carbides present in the layer C1. Thus, depending on the temperature and the duration of the heating, the tantalum carbide TaC of the layer C1 will be decomposed mainly tantalum carbide Ta ₂ C, then tantalum tantalum carbon having Ta ₂ C grain boundaries. Thus, the surface of the part (the so-called "surface layer" below) is free of tantalum carbide of the TaC type, but, as the decomposition begins near the surface, the thickness of this superficial layer can correspond to the thickness of the layer C1 of the carbonaceous multilayer or to an upper part of the layer C1.

• Ta₂C/ Ta sat. C + Ta₂C (c'est-à-dire avec, dans la couche superficielle, du Ta₂C et, dans la partie intermédiaire de la pièce, une couche de Ta saturé en carbone avec du Ta₂C aux joints de grain) ;
• Ta₂C/TaC/Ta₂C/Ta sat. C + Ta₂C (c'est-à-dire avec, dans la couche superficielle, du Ta₂C, et, dans la partie intermédiaire de la pièce, une couche de TaC, une couche de Ta₂C et une couche de Ta saturé en carbone avec du Ta₂C aux joints de grain) ; ou bien encore
• Ta saturé en carbone + Ta₂C /Ta₂C/Ta saturé en carbone + Ta₂C (c'est-à-dire avec, dans la couche superficielle, du Ta saturé en carbone avec du Ta₂C aux joints de grain, et, dans la partie intermédiaire de la pièce, une couche de Ta₂C et une couche de Ta saturé en carbone avec du Ta₂C aux joints de grain) ;

ces structures multicouches étant sur une partie centrale en tantale ou en un alliage de tantale.The method which is the subject of the invention makes it possible, by the same sequence of steps, to cement a part while choosing the structure and the chemical composition of the superficial layer of the cemented part obtained at the end of the process, without having to use chemical treatment with acids or mechanical treatment of the surface of the room. For example, for a tantalum piece, it will be possible to obtain, at the end of the process steps, a surface layer of Ta ₂ C tantalum carbide or carbon-saturated tantalum having Ta ₂ C at the joints. of grain. It is thus possible to obtain multilayer structures of the type, for example:

• Ta ₂ C / Ta sat. C + Ta ₂ C (ie with Ta ₂ C in the surface layer and a layer of carbon-saturated Ta with Ta ₂ C at the grain boundaries in the intermediate part of the part) );
• Ta ₂ C / TaC / Ta ₂ C / Ta sat. C + Ta ₂ C (that is, with Ta ₂ C in the surface layer), and in the intermediate part of the part, a layer of TaC, a layer of Ta ₂ C and a layer of Your saturated carbon with Ta ₂ C at grain boundaries); or even
• It is saturated with carbon + Ta ₂ C / Ta ₂ C / Ta saturated with carbon + Ta ₂ C (that is to say with, in the superficial layer, carbon saturated Ta with Ta ₂ C at grain boundaries and, in the intermediate part of the part, a layer of Ta ₂ C and a layer of carbon-saturated Ta with Ta ₂ C at the grain boundaries);

these multilayer structures being on a central portion of tantalum or a tantalum alloy.

One can also simply have a surface layer Ta sat. C + Ta ₂ C on a central part made of tantalum or a tantalum alloy.

It should be noted that in the examples cited above, the layer Ta sat. C + Ta ₂ C can also be a Ta sat layer. C, if the degree of carbon saturation of the tantalum layer is lower.

It should be noted that in the document [2] is described a method comprising the formation of carbide layers on the surface of a tantalum or tantalum alloy piece, followed by the application of a heat treatment which is implemented to fuel the room in its entirety. Thus, in contrast to the process which is the subject of the invention, where it is desired to keep tantalum or tantalum alloy in the center of the piece, the method described in document [2] aims at producing a saturated piece of carbon. (piece "Your sat C" or "Your sat C + Ta ₂ C") in all its thickness. In addition, the process described does not make it possible to obtain complex multilayer structures of the "low carbon / high carbon layer / low carbon layer" type on a tantalum or tantalum alloy core, as for example those illustrated in FIG. in the figures 5b and 6b hereinafter (that is Ta ₂ C / TaC / Ta ₂ C / Ta sat C + Ta ₂ C / core and Ta sat C + Ta ₂ C / Ta ₂ C / Ta sat C + Ta ₂ C / heart).

In the context of the present invention, it is considered that a tantalum alloy corresponds to an alloy comprising at least 90% by weight of tantalum. Furthermore, it it is a metal alloy, that is to say a mixture of tantalum with another metal. It may for example be a TaW alloy.

1. i) introduire la pièce dans le four;
2. ii) mettre sous vide le four; et
3. iii) chauffer progressivement le four jusqu'à atteindre une température de travail comprise entre 1500 et 1700°C.

Preferably, step a) comprises the operations of:

1. i) insert the piece into the oven;
2. ii) evacuate the furnace; and
3. iii) gradually heat the oven to reach a working temperature between 1500 and 1700 ° C.

Preferably, step b) comprises injecting, preferably continuously, the carbonaceous gas source into the furnace at a flow rate of between 1 and 100 Lh ^-1 and, preferably, at a lower injection pressure. or equal to 10 mbar. The duration of the injection is a function of the amount of carbon that it is desired to introduce into the peripheral part of the tantalum or tantalum alloy part. This time depends on the injection parameters of the carbon source, the surface of the part, as well as the thickness and type of carbon multilayer that is desired.

Preferably, the injection of the gaseous carbon source in step b) is carried out at an injection pressure of 5 mbar for a flow rate of 20 Lh ^-1 and in a furnace heated to a temperature of 1600 ° C.

Preferably, the gaseous carbon source used in step b) is ethylene. The choice of ethylene has the advantage of allowing a low carbon input and limiting the formation of any soot occurring during the use of higher carbon gases, such as acetylene for example.

Step c) is intended to stop the formation of the carbonaceous multilayer; in other words, we seek by this step to stop the carbon contribution in the room. Preferably, step c) comprises an injection of nitrogen gas into the oven at a pressure of 1 bar, which makes it possible to obtain rapid cooling of the part.

1. i) placer la pièce dans une cavité fermée dont les parois sont en un matériau attirant le carbone, l'oxygène et l'azote (le matériau choisi doit bien évidemment supporter les températures de traitement régnant dans le four), ledit matériau étant de préférence en tantale ; et
2. ii) purger la cavité à l'aide d'un gaz inerte de manière à évacuer du four tout gaz susceptible de contenir au moins l'un des éléments atomiques choisis parmi le carbone, l'oxygène et l'azote.

Preferably, step d) comprises the operations of:

1. i) place the piece in a closed cavity whose walls are made of a material attracting carbon, oxygen and nitrogen (the chosen material must be of course, withstanding the processing temperatures prevailing in the furnace), said material preferably being tantalum; and
2. ii) purge the cavity with an inert gas so as to evacuate any oven gas likely to contain at least one of the atomic elements selected from carbon, oxygen and nitrogen.

Step e) comprises heating the room to a temperature sufficient to allow the diffusion of the carbon present in the layer C1 of the carbonaceous multilayer in the direction of the layers C2 and C3. Preferably, step e) comprises heating the oven to a temperature of 1600 ° C and a pressure of 10 ^-2 mbar.

In step f), the cooling is carried out under vacuum in order to protect the tantalum or tantalum alloy part from any traces of residual pollution of the furnace which could be drawn towards the part if a refurbishment was carried out. high temperature oven pressure.

The method which is the subject of the invention comprises numerous advantages.

Firstly, the carbon input in the peripheral part of the part is controlled and mastered, since it comes solely from the gaseous carbon source used during step b) of the process which is the subject of the invention the carbon supply is then prevented by steps c) and d) of the process. Thus, even if there is carbon remaining on the oven walls (in the form of soot, for example, or simply if a furnace with carbon walls is used) and carbon is found in the atmosphere of the oven in step e) because of the heating of the oven, it will be trapped by the protective device and will not be introduced into the room. It is thus possible to obtain a cementation on a controlled thickness of the peripheral part of the part, while preserving in the central part of the part the properties of the original metal and having on the surface a superficial layer which does not contain of tantalum carbide TaC.

Furthermore, the use of a low pressure carburization method (steps a) and b) of the process makes it possible to work at a lower temperature than with other known cementation methods, and the control of the carbon input. allows optimize processing times, which ultimately saves time, energy and consumables.

Hard-to-use chemicals are not used to remove TaC-type tantalum carbide from the surface layer of the part and there is no oxygen and nitrogen pollution in the treated part .

Finally, the method which is the subject of the invention can be used to process parts having complex geometries and / or having singularities (holes of small diameters, etc.).

Other features and advantages of the invention will emerge from the additional description which follows and which relates to examples of implementation of the manufacturing method according to the invention.

It goes without saying that this additional description is given only as an illustration of the subject of the invention and should in no way be interpreted as a limitation of this object.

BRIEF DESCRIPTION OF THE DRAWINGS

• The figure 1a represents a schematic cross-sectional view of a portion of a tantalum piece obtained at the end of step b) of the method that is the subject of the invention according to a particular embodiment (1 hour of cementation at 1600 ° C.) and showing the layers of carbides created on the surface of the piece.
• The figure 1b represents a snapshot obtained by scanning electron microscopy (SEM) of the piece illustrated in FIG. figure 1a .
• The Figures 2a and 2b represent the cementation time as a function of time at different temperatures for the growth of TaC layers ( figure 2a ) and for the growth of Ta ₂ C layers ( figure 2b ).
• The figure 3a represents a schematic cross-sectional view of a portion of a tantalum piece obtained according to a particular embodiment of the method that is the subject of the invention (1 hour of carburizing at 1600 ° C., cooling and 1 hour of vacuum heating at 1600 ° C) and showing the layers of carbides created on the surface of the piece.
• The figure 3b represents a snapshot obtained by SEM of the piece illustrated in the figure 3a .
• The figure 4a represents a diagrammatic sectional view of a portion of a tantalum piece obtained according to a particular embodiment of the process which is the subject of the invention (with 1 hour of cementation at 1600 ° C. in step b) and 6 hours of vacuum heating at 1600 ° C in step e).
• The Figure 4b and 4c represent respectively a snapshot obtained by SEM of the piece illustrated in the figure 4a according to two different magnifications. It should be noted that in the figure 4c , the piece was etched to reveal the presence and location of Ta ₂ C precipitates (black spots).
• The figure 5a represents a schematic cross-sectional view of a portion of a tantalum piece obtained according to a particular embodiment of the process which is the subject of the invention (with 2 hours of cementation at 1600 ° C. in step b) and 30 minutes of heating under vacuum at 1600 ° C in step e)).
• The figure 5b represents a snapshot obtained by SEM of the piece illustrated in the figure 6a .
• The figure 6a represents a schematic cross-sectional view of a portion of a tantalum piece obtained according to a particular embodiment of the process that is the subject of the invention (with 2 hours of cementation at 1600 ° C. in step b) and 6 hours of heating under vacuum at 1600 ° C in step e)).
• The figure 6b represents a snapshot obtained by SEM of the piece illustrated in the figure 6a .

It should be noted that the figures above, the central part of the piece is never shown.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

The method which is the subject of the invention makes it possible to control the cementation of a tantalum or tantalum alloy part, while choosing the nature and the crystallographic structure of the surface layer of the part. It makes it possible to obtain, at the surface of the part, a surface layer of tantalum carbide of Ta ₂ C type with an underlying layer of tantalum carbide of the TaC type or of carbon-saturated tantalum with Ta ₂ C at the grain boundaries, a mixed surface layer of tantalum saturated with carbon with Ta ₂ C at grain boundaries, or even a superficial layer of carbon-saturated tantalum with an underlying Ta ₂ C-type tantalum carbide layer, while controlling the thickness of this superficial layer.

As we said above, the duration of the heating in step b) of the method that is the subject of the invention depends on the amount of carbon that it is desired to bring to the part. The duration of the heating in step e) is, in turn, depending on the nature of the layer that is desired to obtain on the surface, as well as the thickness that it is desired that it has. By playing on these parameters, it is possible to obtain monolayer structures (a surface layer of Ta ₂ C, Ta sat, C + Ta ₂ C or Ta sat C on a tantalum or tantalum alloy core) or multilayer structures (Ta ₂ C / TaC / Ta ₂ C / Ta sat layers C + Ta ₂ C, Ta sat layers C + Ta ₂ C / Ta ₂ C / Ta sat layers C + Ta ₂ C, etc. , on a heart made of tantalum or an alloy of tantalum). Obtaining these different structures makes it possible to reinforce the hardness and / or the resistance to corrosion of the part, so as to make it compatible with its end use.

To illustrate the invention, we will now describe a preferred embodiment of the method which is the subject of the invention.

A tantalum piece, for example a crucible having a diameter of 100 mm, is used for a thickness of 1.5 mm and a height of 150 mm.

The part to be treated is installed in the enclosure of an oven, for example a furnace of mark BMI bearing the reference BMICRO.

Then, the oven chamber is evacuated until a pressure of 10 ^-2 ± 0.01 mbar is reached.

After stabilization of the pressure, the enclosure is heated with a ramp of 30 ° C / min, until reaching 1600 ° C ± 1%.

The work is then cemented by injecting into the chamber a fuel gas under a low pressure (pressure less than about ten millibars) for a predetermined period. In this example, ethylene is injected (C ₂ H ₆₎ in the enclosure under a pressure of 5 ± 1 mbar and at a controlled flow rate of 20 L / h for 1 hour.

The part is then cooled, for example by means of nitrogen injected into the chamber of the oven at a pressure of 1 bar for a period of 90 minutes.

Thus, in the peripheral portion of the tantalum piece, a carbon-based multilayer 1 comprising a surface layer C1 of tantalum carbide of the TaC type, an underlying layer C2 of tantalum carbide of Ta ₂ C type and a layer under C3 in carbon-saturated tantalum with precipitates of Ta ₂ C at the grain boundaries ( Figures 1a and 1b ).

où W représente l'épaisseur de la couche de carbure (en µm), t la durée de maintien (en minutes) et k le coefficient de croissance (µm².min^-1). Par analogie, on applique la même formule pour la formation du multicouche carboné 1. La vitesse de formation du multicouche carboné dépend également de la température de cémentation. Cette vitesse de formation croit exponentiellement avec la température.The thickness of the carbonaceous multilayer 1 (and therefore the total quantity of carbon introduced into the part) depends on the holding time of the tantalum part under the flow of fuel gas ( Figures 2a and 2b ). Indeed, in known manner, the growth of tantalum carbide layers follows the parabolic law $W=\sqrt{\mathit{kt}}$

where W represents the thickness of the carbide layer (in μm), t the holding time (in minutes) and k the growth coefficient (μm ² .min ^-1 ). By analogy, the same formula is applied for the formation of the carbonaceous multilayer 1. The rate of formation of the carbonaceous multilayer also depends on the carburising temperature. This formation rate exponentially increases with temperature.

The piece thus treated is then cut off from any source of carbon, as well as any pollutants. This step is necessary if one wants to avoid the phenomena of pollution of the tantalum during the stage of diffusion and to control the quantity of carbon present in the room. Indeed, tantalum is a very hot reactive element vis-à-vis atoms such as carbon, oxygen and nitrogen and these elements can for example be in molecules adsorbed on the walls of the oven enclosure.

For this, according to a preferred embodiment of the method according to the invention, the piece is placed in a cavity (for example formed by depositing a bell on a support, the bell and the support being both in tantalum) which is placed in the oven enclosure. This makes it possible to trap polluting elements (O, N ₂ , etc.), as well as any carbon atoms present on the walls of the furnace enclosure, before they come into contact with the room. This also makes it possible to slow gas exchange between the furnace chamber and the workpiece, which is favorable for the carbon diffusion process.

A double pumping of the furnace enclosure may optionally be carried out by performing an intermediate purge with nitrogen (pressure of 10 ^-2 +/- 0.01 mbar) in order to evacuate any pollutant.

Then, we proceed to the heating of the room. The vacuum heating in step e) will allow the carbon present in the layer C1 of the carbonaceous multilayer 1 to diffuse to the layers C2 and C3 of the multilayer.

• le type de structure que l'on souhaite obtenir à l'issue du procédé ;
• l'épaisseur du multicouche formé lors de l'étape de cémentation ;
• l'épaisseur de la pièce.

The heating time of the assembly formed by the workpiece and the protective device depends on three parameters:

• the type of structure that one wishes to obtain at the end of the process;
• the thickness of the multilayer formed during the carburizing step;
• the thickness of the room.

The assembly formed by the part and the protective device (cavity) is heated at 30 ° C / minute until the desired treatment temperature. We choose here to use the same temperature as that used for carburizing, that is to say 1600 ° C +/- 1%.

At the end of step b) (after cementation), the tantalum piece had on the surface a carbonaceous multilayer 1 having a surface layer C1 in TaC, an underlying layer C2 in Ta ₂ C and an underlying layer C3 in carbon saturated tantalum with precipitates of Ta ₂ C at the grain boundaries. During the heating in step e), the carbon diffuses from the surface layer C1 to TaC (the layer richest in carbon) towards the layer C2 in Ta ₂ C, and from the layer C2 in Ta ₂ C to the layer C3 in your sat. C + Ta ₂ C. This cascade carbon diffusion causes the reduction in thickness of the layer of TaC the benefit of the Ta layer ₂ C. It is then possible to completely disappear layer of TaC in favor of a single layer of Ta ₂ C at the surface of the piece. If the heating is continued, the layer of Ta ₂ C also decomposes to disappear completely. Only carbon-saturated tantalum with precipitates of Ta ₂ C at the grain boundaries remains on the surface.

Different structures that can be obtained by varying the duration of the heating in step b) and / or step e) are illustrated in the following figures.

As specified above, after heating the tantalum piece for 1 hour at 1600 ° C. in step b), a carbonaceous multilayer 1 having a layer C1 in TaC, a layer C2 in Ta ₂ C and a layer is obtained. C3 in Ta sat. C + Ta ₂ C ( Figures 1a and 1b ).

If it is then subjected to the other steps of the process object of the invention, including 1h of heating under vacuum at 1600 ° C in step e) after being isolated from any source of carbon, a surface layer 2 is obtained in Ta ₂ C on an underlying layer 3 in Ta sat. C + Ta ₂ C ( Figures 3a and 3b ).

If, on the other hand, the part provided with the carbonaceous multilayer is subjected to vacuum heating for 6 h at 1600 ° C. in step e), a carbon-saturated tantalum layer 2 with Ta ₂ C precipitates is obtained. at the grain boundaries ( Figures 4a, 4b and 4c , the precipitates being visible in black on the figure 4c ). Here, it can be supposed that the carbon diffusion is such that the layers C1, C2 and C3 of the carbonaceous multilayer are transformed into the surface layer 2.

According to another example, if the piece is subjected to carburization by heating under vacuum at 1600 ° C. for 2 hours in step b) and heating under vacuum at 1600 ° C. for 30 minutes in step e) a part having a surface layer 2 in Ta ₂ C, a first sub-layer 3 in TaC, a second sub-layer 4 in Ta ₂ C and a third sub-layer 5 in Ta sat are obtained. C + Ta ₂ C ( Figures 5a and 5b ).

If, on the contrary, it is subjected to carburization by heating under vacuum at 1600 ° C for 2 hours in step b) and heating under vacuum at 1600 ° C for 6 hours in step e), a superficial layer is obtained. 2 in Ta sat. C, a first sub-layer 3 Ta ₂ C and a second sub-layer 4 Ta sat. C + Ta ₂ C ( Figures 6a and 6b ).

REFERENCES CITED

1. [1] US 5,916,377
2. [2] US5,383,981

Claims (7)

1. A process for treating a piece of tantalum or of a tantalum alloy, the process comprising the steps of:

   a) placing the piece in a furnace and heating the furnace under vacuum at a temperature at least equal to 1,400°C;

   b) forming a carbon multilayer (1) in the peripheral part of the piece, by injecting, in the heated furnace, a gas carbon source at a pressure at most equal to 10 mbar, the carbon multilayer comprising at least one layer C1 of tantalum carbide, which is located at the surface of the piece, and two underlying layers C2 and C3 each comprising a carbon content which is different and lower than a carbon content of the layer C1;

   c) stopping the formation of the carbon multilayer (1) by cooling the piece;

   d) placing around the piece a protecting device for trapping carbon, oxygen and nitrogen to protect the piece from carbon as well as possible oxygen and nitrogen traces present in the furnace;

   e) causing a diffusion of all or part of carbon present in the layer C1 towards the layers C2 and C3, by heating the furnace under vacuum, the piece being held in the protecting device; and

   f) stopping the diffusion of carbon in the piece by cooling the piece under vacuum before carbon present in the carbon multilayer reaches the centre part of the piece;

   whereby there is obtained a piece whose surface is free from tantalum as TaC, whose centre part is free from carbon and whose part which is located between the surface and the centre part comprises tantalum and carbon.
2. The process according to claim 1, wherein step d) comprises:

   i) placing the piece in a closed cavity having walls made of a material attracting carbon, oxygen and nitrogen, the material being preferably of tantalum; and

   ii) draining the cavity using an inert gas so as to discharge from the furnace any gas likely to contain at least one of atomic elements chosen from carbon, oxygen and nitrogen.
3. The process according to claim 1 or claim 2, wherein step a) comprises:

   i) introducing the piece into the furnace;

   ii) putting the furnace under vacuum; and

   iii) gradually heating the furnace until a working temperature between 1,500 and 1,700°C is reached.
4. The process according to any of claims 1 to 3, wherein step b) comprises injecting the gas carbon source in the furnace at a flow rate between 1 and 100 L.h^-1 and at an injection pressure lower than or equal to 10 mbar.
5. The process according to claim 4, wherein the injection of the gas carbon source in step b) is made at an injection pressure of 5 mbar for a flow rate of 20 L.h^-1 and in a furnace heated at a temperature of 1,600°C.
6. The process according to any of claims 1 to 5, wherein step e) comprises heating the furnace at a temperature of 1,600°C and at a pressure of 10^-2 mbar.
7. The process according to any of claims 1 to 6, wherein the gas carbon source used in step b) is ethylene.

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